Fiber optic communications is being extended for small business and home use. This provides increased bandwidth and features over that of wired or electrical connections. This is known as Fiber to the Home or FTTH or FTTx where x is home or business or some other user or subscriber or equipment. A most economic method of providing multiple houses in an area is know as Passive Optical Networks or PON. PON makes maximum use of completely passive (nonelectronic) optical devices for coupling multiple homes from a single fiber source feed. PON is seen as a way to amortize the expensive electronics over multiple users in a reliable and efficient way.
The PON method creates a situation where the end user is not exclusively connected to a single wire or fiber from the service provider's source. This is unlike the traditional telecommunications network where every subscriber is connected to a single signal circuit from the central office. This creates a problem for the operation and maintenance of PON systems in that if there is a cable cut or some other failure it cannot be determined from a central location. The failure must further be researched at the site of the PON ‘coupler’, alternately known as a ‘splitter’.
This produces a testing or monitoring blindness that can only be resolved by physically separating each subscriber and applying test signals to determine the location and perhaps distance of a cable break.
A traditional method of determining cable fault locations in fiber optical systems is to use optical time domain reflectometery (OTDR). This method sends a signal along a fiber and detects return signals from various components on the optical path including cable breaks. This method can be carried out from a central office or along the fiber route.
A major problem with PON systems is that the coupler, which splits the signal from the source to the individual subscriber, also splits any diagnostic or OTDR signal. The result is that cable faults beyond the PON coupler are difficult or impossible to resolve using OTDR methods. If this signal loss can be overcome it is done with expensive and sensitive equipment, and accurate network records must be kept noting changes such as adding or deleting equipment or subscribers, which has a major impact on the return OTDR signal. Even with this information, practical PON OTDR may be impossible.
Referring to FIG. 7, typically a PON coupler connects a single source fiber to between 4 and 32 subscribers. The signal loss for an OTDR must account for the forward signal and reverse (reflection) loss through the coupler. For a subscriber PON split of 32, there is a minimum of approximately 38 dB of loss for the return test signal. Cable and splice losses make this worse. Any subscriber who's cable is damaged or is not terminated may swamp this low level return signal making it impossible to determine outages beyond the coupler. Test equipment manufactures suggest it is possible to use OTDR in PON systems but they state that this requires detailed records and measurements of the specific installations, cable lengths and characteristics and coupler measurements and any changes in customer or environmental conditions. These requirements are very unlikely to be implemented in any practical system. Also it is known that record keeping and data collection of changing systems introduce errors, which would further make the use of OTDR and couplers ineffective.
Another problem with OTDR methods on PON systems is that they use high-powered laser pulses to generate the test signal. This has a tendency to blind or interfere with normal communications and data being carried out on the fiber, creating multiple alarms and subscriber outages. This has a large impact on a PON system where multiple users are on the system at the same time.
One solution is to connect the OTDR to each subscriber fiber individually as shown in FIG. 2, beyond the coupler. This is time consuming, expensive and interrupts each subscriber individually for the test time.
In fiber optic systems, a coupler can be used to split or combine optical power. Couplers can also be used in these systems to divide the frequencies or wavelengths of individual fiber optic signals. In these systems the term Wave Length Division Multiplexing (WDM) or Dense WDM (DWDM) is used. The system can combine or divide by wavelength, as shown in FIG. 12, having wavelengths L1 to LN.
It is realized that a PON (Passive) system can be made of Passive WDM components and perform similar functions to enable utility. Like a PON system the test access of a WDM system typically requires the disconnection of individual fibers to gain test access. Like a PON system accessing individual fibers by disconnection is a manual process, which causes outages to wavelengths or groups of wavelengths.